The AROS distribution Icaros Desktop has made its next step towards compatibility with legacy Amiga workbench applications, including an entire AROS enviroment compiled for the classic Amiga platform, which is almost binary compatible with the original Amiga OS 3.1 (and its extensions). When the user needs an old program, he or she only has to fire up the AROS M68K environment and run the application. The Amiga virtual machine can optionally be set to run at startup like a system service.

OS4 does have a native port of MUI which is distributed with it, but it's not the "standard" toolkit. You'll find that most OS4-only programs are Reaction-based, whereas programs which are ported from other Amiga-ish platforms tend to use MUI. Also, if you're writing some software for OS4 or any of the platforms and want to port it to the others, your best bet is to start with MUI as it's common across AROS, Morphos, OS4 and classic.